1. Field of the Invention
The present invention relates to a magneto-optical recording apparatus for recording information on a magneto-optical recording medium with an objective lens having high NA. The present invention also relates to a magneto-optical head to be used for the magneto-optical recording apparatus.
2. Related Art
In recent years, in response to the advance of multimedia, the optical recording medium has been used, on which a large amount of data can be recorded at a high density, and recording and reproduction can be performed at a high speed. Such an optical recording medium includes read-only media such as compact disks on which information is previously recorded as pits on the disk during formation of the disk, write-once media such as CD-R on which recording can be performed only once, and rewritable media on which data can be rewritten any number of times by the magneto-optical recording system or the phase-change recording system.
Among the optical recording media as described above, a magneto-optical recording medium is principally used in a field in which a high transfer rate is required. When the magneto-optical recording medium is subjected to recording and reproduction, a laser beam is converged with a lens up to the diffraction limit so that the magneto-optical recording medium is irradiated therewith. The light spot, which is formed on the magneto-optical recording medium as a result of the irradiation with the laser beam, has a size of about xcex/NA provided that xcex represents the wavelength of the laser beam and NA represents the numerical aperture of the lens.
In order to record or reproduce a pattern having a higher density, i.e., having a smaller size, it is necessary to use a smaller laser beam spot. In order to decrease the size of the light spot, it is conceived that the wavelength (xcex) of the laser beam is decreased or the numerical aperture (NA) of the lens is increased according to the expression described above. The numerical aperture (NA) of the lens is represented by NA=sin xcex8 provided that xcex8 represents the focusing half angle of the lens, and numerical aperture (NA) of the lens has a value smaller than 1. The objective lens, which is used in an optical recording apparatus commercially available at present, has NA of about 0.6 at most. The reason why the value of NA is restricted to be about 0.6 is as follows. That is, if the NA is increased, the astigmatism and the aberration generated when the optical axis of the objective lens is inclined with respect to the disk surface are increased. In order to solve this problem, it is necessary to thin the thickness of the transparent substrate of the disk through which the light beam is transmitted before arrival at the magneto-optical recording layer of the magneto-optical recording medium. However, if the thickness of the substrate is made thin, then the surface fluctuation of the disk is increased due to the decrease in rigidity of the substrate, and the substrate is inclined with respect to the optical axis of the objective lens. For this reason, the aberration as described above is generated.
In view of the above, it is conceived that the light beam is allowed to come into the disk from the side opposite to the substrate of the disk, i.e., from the side of the film surface, when the disk is irradiated with the light beam. Even if the film surface incidence system is adopted as described above, various problems still remain when the objective lens having high NA is used.
For example, a magneto-optical head, in which an optical system and a magnetic coil are integrated into one unit, is known to be used in a magneto-optical recording apparatus. Such a magneto-optical head is effective when information is recorded in the magneto-optical recording layer by using a lens having high NA. However, the magnetic characteristics are deteriorated especially when the recording is performed by applying a modulated magnetic field having a high frequency, because the magnetic coil is heated by the radiated light beam. Japanese Patent Application Laid-Open No. 11-316986 discloses an optical head which is provided with a thermal conduction film between a coil and a coil support member. This patent document discloses the following technique. That is, the thermal conduction film has a coefficient of thermal conductivity which is not less than at least several times a coefficient of thermal conductivity of the coil support member. Therefore, the heat, which is generated in the coil, can be efficiently conducted to an objective lens via the thermal conduction film to suppress the increase of temperature of the coil. However, if the heat is conducted to the objective lens, a problem arises such that the optical characteristics of the lens are deteriorated due to the thermal expansion of the lens.
Usually, the magneto-optical head includes two types of lenses which are provided in combination to be used as an objective lens system. When the lenses each having high NA are used for the objective lenses, the wave front aberration readily tends to appear due to any discrepancy between the optical axes of the lenses. The wave front aberration as described above decreases the reproduced signal characteristics. Therefore, it is necessary that the optical axes of the two types of the lenses of the objective lens system are mutually adjusted highly accurately.
The present invention has been made in order to solve the problems involved in the conventional technique as described above, a first object of which is to provide a magneto-optical head and a magneto-optical recording apparatus based on the use of the magneto-optical head which makes it possible to effectively release heat from a coil without deteriorating the optical characteristics of an objective lens.
A second object of the present invention is to provide a magneto-optical head and a magneto-optical recording apparatus based on the use of the magneto-optical head which makes it possible to effectively avoid any occurrence of aberration by highly accurately adjusting optical axes of two lenses which construct an objective lens system of the magneto-optical head.
According to a first aspect of the present invention, there is provided a magneto-optical head to be used for recording information on an information-recording medium, comprising:
a lens which collects a light beam onto the information-recording medium;
a substrate which has a first surface and a second surface parallel to one another and which supports the lens on the first surface;
a magnetic coil which is provided on the second surface of the substrate; and
a heat-releasing member which is provided on the second surface of the substrate and which releases heat generated by the magnetic coil.
In the magneto-optical head according to the present invention, the heat-releasing member is provided on the second surface. Accordingly, even when the magnetic coil is heated by the radiation of the light beam, the heat is not released to the first surface side of the substrate on which the lens is provided. Therefore, it is possible to avoid the thermal expansion of the lens, especially of the objective lens having high NA of 0.6xe2x89xa6NA, and it is possible to appropriately maintain the optical characteristics of the lens. On the other hand, the heat-releasing member, which is provided on the second surface, makes it possible to release the heat generated by the magnetic coil and the vicinity thereof, to the space between the magneto-optical head and the information-recording medium. Especially, the information-recording medium usually makes the rotating movement at a linear velocity of not less than 1 m/s with respect to the magneto-optical head. Therefore, the heat-releasing member is efficiently cooled by the air stream which is generated by the rotating movement.
In the present invention, it is desirable that the heat-releasing member is provided at a position which is nearer to the information-recording medium as compared with the magnetic coil in an optical axis direction of the lens. Especially, when the heat-releasing member is exposed on the magneto-optical head so that the heat-releasing member is opposed to the information-recording medium, the heat-releasing member is cooled more effectively by the air stream which is generated by the rotating movement of the information-recording medium. Alternatively, the heat-releasing member may be provided at the outside of the magnetic coil.
In the present invention, the substrate may be a glass flat plate. When the glass flat plate, in which the first surface and the second surface are parallel to one another, is used, if the light-outgoing plane of the objective lens is flat, then it is easy to arrange the glass substrate correctly perpendicularly to the optical axis of the lens. Accordingly, it is easy to adjust the optical path for the light beam after passing through the glass flat plate.
In order to obtain an appropriate light beam by using the magneto-optical head, it is necessary to control the thickness of an adhesive layer to be constant, when the lens is bonded to the support substrate. When a support pad, which serves as a spacer, is provided between the lens and the support substrate, it is possible to control the thickness of the adhesive layer to be constant. Accordingly, it is easy to arrange the support substrate correctly perpendicularly with respect to the optical axis of the lens. Thus, it is possible to obtain an appropriate light beam on the magneto-optical disk. It is preferable that the support pad is composed of metal, for the following reason. That is, when the support pad is made of metal, the support pad can be simultaneously formed in the step of plating the terminal of the coil. When the pad is bonded by an adhesive, the portion (light-transmitting section), which is disposed between the disk substrate surface (second surface) and the end surface on the light-outgoing side of the lens, may be filled with the adhesive. In this arrangement, it is desirable for the adhesive to select one having a refractive index which is approximate to refractive indexes of the lens and the substrate. When the gap is formed without filling the portion with the adhesive, a reflection-preventive film may be formed on the end surface on the light-outgoing side of the lens and the first surface of the substrate, in order to avoid any interface reflection of the light beam, which would be otherwise caused between the glass and the gap.
A projection or a protective pad may be provided on the second surface of the substrate. Even when any dust adheres to the disk, the projection or the protective pad protects the magnetic coil and the light-transmitting section provided at the center of the magnetic coil. It is possible to provide the magneto-optical head having high durability. The protective pad may be formed of a variety of resin materials.
When the surface of the projection is formed with a protective film such as those composed of, for example, silicon oxide, silicon nitride, zirconia, and diamond-like carbon, it is possible to provide the magneto-optical head having more excellent durability.
In the magneto-optical head according to the present invention, it is desirable that the support substrate has an areal size which is larger than that of the lens in a direction along a surface of the substrate. When such an arrangement is adopted, the terminal of the coil can be led from the first surface of the substrate. The terminal of the coil does not protrude from the second surface of the substrate which is disposed closely to the disk surface. The coil may be stacked in two or more layers in the optical axis direction. In order to stack the coil in two or more layers, for example, the following procedure may be adopted. That is, a helical coil is formed from the outer side to the inner side on an identical surface (layer) (formation of first layer), and then the inner circumferential end is bent upwardly (or downwardly) in the optical axis direction to allow the coil to extend. Further, the coil is bent in the outer circumferential direction to allow the coil to extend in a helical form (formation of second layer). The outer i.e., at the center of the light spot on the disk. On the contrary, when A is larger than B, it is possible to intensify the magnetic field up to the central position of the coil. However, if A is extremely larger than B, the effect as described above is reduced. Therefore, it is especially preferable to satisfy 1.2xe2x89xa6A/Bxe2x89xa63.2.
The soft magnetic layer preferably has an oblong configuration to cover the coil, for the following reason. That is, it is intended to facilitate the control of the magnetic permeability of the soft magnetic layer formed on the disk surface or the opposite surface of the coil. Alternatively, an equivalent effect is, also obtained by dividing the soft magnetic layer into a plurality of individuals to form a strip-shaped configuration.
According to a second aspect of the present invention, there is provided a magneto-optical recording apparatus for recording information on an information-recording medium, comprising:
a light source;
a magneto-optical head with which a light beam from the light source is radiated onto the information-recording medium to record the information; and
a detecting system which detects the light beam returning from the information-recording medium, wherein the magneto-optical head comprises:
a lens which collects the light beam onto the circumferential end of the second layer of the coil can be led to the first surface of the substrate via a through-hole previously formed through the substrate.
A first soft magnetic layer, which is formed with an aperture for transmitting the light beam, may be provided between the coil and the second surface of the substrate. The soft magnetic layer may be formed over a substantially entire surface of the support substrate except for the light-transmitting section which is disposed at the center of the coil. When the soft magnetic layer is provided, the heat, which is generated by the coil, is easily diffused via the soft magnetic layer. The soft magnetic layer also plays a role to increase the magnetic field which is generated by the coil. Further, in addition to the first soft magnetic layer, a second soft magnetic layer, which is formed with an aperture for transmitting the light beam, may be further provided on a side of the disk of the coil.
When the first and second soft magnetic layers are provided, it is preferable that A is larger than B provided that A represents an areal size of the aperture of the second soft magnetic layer for transmitting the light beam and B represents an areal size of the aperture of the first soft magnetic layer for transmitting the light beam. If A and B are substantially equal to one another, the magnetic field is increased in the vicinity of the coil. The magnetic field is not increased so much at the central position of the coil, information-recording medium;
a substrate which has a first surface and a second surface parallel to one another and which supports the lens on the first surface;
a magnetic coil which is provided on the second surface of the substrate; and
a heat-releasing member which is provided on the second surface of the substrate and which releases heat generated by the magnetic coil.
It is desirable that the magneto-optical recording apparatus according to the present invention is provided with a two-dimensional actuator which simultaneously performs focus servo and tracking servo. In this arrangement, it is preferable that the two-dimensional actuator is installed with the substrate provided with the magnetic coil. When this structure is adopted, the magnetic field can be stably applied to the recording film on which the light beam is collected, even when the light-collecting position of the light beam is changed by moving the lens while following the eccentric movement and the surface fluctuation of the disk. The numerical aperture (NA) of the lens may be 0.7 to 0.95. In order to perform the super high density recording, a solid immersion lens having NA of not less than 1 may be used.
The magneto-optical head described above may be installed on a floating (flying) slider to provide a floating type head. In this case, a groove for the floating action may be formed on the surface of the slider.
Those usable as the information-recording medium include a magneto-optical recording medium in which at least a reflective film, a recording layer, and a transparent dielectric layer are stacked in this order on a substrate provided with lands or grooves, for example, in order to perform the groove recording or the land recording with a track pitch of not more than 0.9 xcexcm. In this specification, it is intended that the term xe2x80x9cmagneto-optical recording apparatusxe2x80x9d refers to an apparatus which has not only the recording function but also the reproducing function.
According to a third aspect of the present invention, there is provided a magneto-optical head to be used for recording information on an information-recording medium, comprising:
a first lens and a second lens which have a common optical axis to collect a light beam onto the information-recording medium;
a first lens holder which holds the first lens;
a second lens holder which holds the second lens;
a first actuator which is provided between the first lens holder and the second lens holder and which relatively moves the first lens holder with respect to the second lens holder in a direction perpendicular to the optical axis of the first lens;
a substrate which has a first surface and a second surface, and which supports the first lens holder and the second lens holder on the first surface; and
a magnetic coil which is provided on the second surface of the substrate.
In the magneto-optical head described above, the optical axes of the first lens and the second lens can be correctly adjusted by relatively moving the first lens holder with respect to the second lens holder by using the first actuator. Therefore, it is possible to avoid any occurrence of the wave front aberration even when a lens having high NA is used for the second lens. The discrepancy between the optical axes of the first lens and the second lens can be detected by detecting a reproduced signal from the information-recording medium while relatively moving the first lens holder with respect to the second lens holder by the first actuator.
When the magneto-optical head is a head of the pickup type, it is preferable that the magneto-optical head further comprises a second actuator which relatively moves the first lens holder with respect to the second lens holder in a direction of the optical axis.
A protective pad may be provided on the second surface of the substrate. In this arrangement, it is preferable that the protective pad covers at least four corners of the second surface of the substrate. The protective pad may cover the whole of the outer circumferential portion of the second surface of the substrate. Alternatively, isolated pads may be provided for only the four corners. The pad may be stuck to the second surface of the substrate. Alternatively, the pad may be fitted into the substrate.
Those usable as the material for the protective pad include, for example, polyethylene, polystyrene, polypropylene, polycarbonate, polyacrylate, polymethacrylate, ABS resin, polyethylene terephthalate, polyacetal, polyarylate, nylon, polyether imide, polyether amide, phenol resin, and fluororesin such as perfluoropolyethylene.
According to a fourth aspect of the present invention, there is provided a magneto-optical recording apparatus for recording information on an information-recording medium, comprising:
a light source;
a magneto-optical head with which a light beam from the light source is radiated onto the information-recording medium to record the information; and
a detecting system which detects the light beam returning from the information-recording medium, wherein the magneto-optical head comprises:
a first lens and a second lens which collect the light beam onto the information-recording medium;
a first lens holder which holds the first lens;
a second lens holder which holds the second lens;
a first actuator which is provided between the first lens holder and the second lens holder and which relatively moves the first lens holder with respect to the second lens holder in a direction perpendicular to an optical axis;
a substrate which has a first surface and a second surface, the second lens being supported on the first surface; and
a magnetic coil which is provided on the second surface of the substrate. The magneto-optical recording apparatus may further comprise a second actuator which relatively moves the first lens holder with respect to the second lens holder in a direction of the optical axis.